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On a returnless fuel system like in the 98+ mustangs the pressure would stay but on a return style pressure drops rather fast. An easy way to check if your injectors are leaking take off the intake tube and open the TB blade. Smell in there if there is a really strong smell of gas then you have a leak.
To me it sounds like you have a bad tune. Do you know what tables your tuner messed with?
I don't have a clue really, he used my SCT Livewire but I had returned it since then, a lot of the functions on it didn't work correctly (such as 1/4mile ET, 0-60) so I wanted my like $600 back.